Did you make this instructable?

I worry the 9V in series to the 3V to equal to 12V will have the power to inflate a large plastic bag.You need a Double Pole Double Throw switch to reverse the fan.See the circuit.... Hope this helps. . . . . . A

I've done some more research and I've ended up connecting the 12v dc fan to an ac-dc adapter and it's working out good. In reversing the fan would i also need a double pole double throw switch in this case?

Yes I will be using a very thin plastic bag. similar to the type that you use to bag produce at the grocery store. I'm going to experiment with other thicknesses also. Is there any other low cost fans that you could recommend? I just used a case fan because it was readily available. I've switched from batteries to an ac-dc adapter. Any concerns with this method?